The present invention relates to an automatic shutdown system for an electrophotographic copying machine.
An electrophotographic copying machine to which the present invention constitutes an improvement generally comprises an endless belt formed with an outer photoconductive surface. The belt is electrostatically charged and a light image of an original document to be copied is radiated onto the belt thereby causing local photconduction and dissipation of the electrostatic charge to form an electrostatic image on the belt which is subsequently developed through the application of a toner substance thereto. The toner image is transferred and fixed to a copy sheet to provide a permanent copy of the original document.
The light image is radiated onto a flat portion of the belt by an imaging system comprising a flash tube to illuminate the document. Although the belt is continuously moving, duration of the flash from the flash tube is so brief that a sharp image is formed regardless of the movement of the belt. A discharge capacitor is connected across the flash tube and is charged from a voltage source. When sufficient charge is accumulated on the capacitor, the tube is fired so that the capacitor is discharged through the tube thereby producing an intense flash of light to illuminate the document and image the belt.
Such a copying machine further typically comprises one or more failure sensors to detect a sheet jam or the similar failure condition and shut down the apparatus in response thereto. However, if the failure occurs after a portion of the belt is electrostatically charged, when operation of the machine is restored after repair of the failure condition and the charged portion of the belt is developed, the charged area will constitute a completely black toner image. This condition results in waste of toner and also an extreme burden on a cleaning unit provided to remove residual toner substance from the belt prior to the subsequent copying operation. In addition, the capacitor must have a very large value of capacitance and be charged to a high voltage in order to provide the required luminous intensity when discharged through the flash tube. If the apparatus is simply shut down in response to a failure condition, a voltage will remain on the capacitor constituting a shock hazzard for repair personnel.